Organic el element and its manufacturing method

ABSTRACT

There is provided an organic EL element having, between a positive electrode and a negative electrode, a lamination structure formed of organic films having a light emitting layer, a hole transport layer adjacent to a positive electrode side of the light emitting layer, and an electron transport layer adjacent to a negative electrode side of the light emitting layer. At least one of the organic films composing the lamination structure includes a metal element having reactivity to oxygen or water.

This application is a Divisional of co-pending application Ser. No.11/587,692 filed on Oct. 26, 2006 and for which priority is claimedunder 35 U.S.C. §120. Application Ser. No. 11/587,692 is the nationalphase of PCT International Application No. PCT/JP2004/006047 filed onApr. 27, 2004 under 35 U.S.C. §371. The entire contents of each of theabove-identified applications are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to an organic electroluminescence element(hereinafter called organic EL element) in general, and moreparticularly relates to an organic EL element having a long life.

RELATED ART

Organic EL elements have properties including spontaneous light, highspeed response and the like, and are expected to be applied to planedisplay devices or the like.

Since the lamination device having a hole transporting organic film andan electron transporting organic film in a laminated manner has beenreported (C. W. Tang and S. A. VanSlyke, Applied Physics Letters, vol.51, 913 (1987)), the organic EL element is attracting attention as alarge area luminous element for emitting light at low voltage of 10 V orless. This conventional organic EL element emits a green light.

FIG. 1 shows a typical structure of laminated organic EL element 10.

In FIG. 1, the organic EL element 10 has a lamination structureincluding a hole transport layer 12 which has the hole transportingorganic film and is formed on a transparent substrate 11 formed of glassor the like carrying a transparent electrode 11A such as ITO(In₂O₃.SnO₂), a light emitting layer 13, and an electron transport layer14 which is formed of an electron transporting organic film. Anelectrode 15 formed of Al or the like is formed on the electrontransport layer 14.

Herein, as shown in FIG. 1, a direct-current driving power source 16 isprovided, and a negative voltage is applied from the direct-currentdriving power source 16 to the electrode 15 contacting with the electrontransport layer 14, and a positive electrode is applied to the electrode11A contacting with the hole transport layer 12, and hence a hole isinjected in the light emitting layer 13 through the hole transport layer12, and an electron is injected through the electron transport layer 14,and by recombination of electron and hole in the light emitting layer13, desired emission occurs in the light emitting layer 13 at wavelengthcorresponding to the energy gap of the light emitting layer 13.

The function of the light emitting layer 13 may be also played by thehole transport layer or electron transport layer, same as in the case oftwo-layer element proposed by Tang and VanSlyke. Further, in view ofobtaining an organic EL element having high emission efficiency, inaddition to a single layer formed of one material, a pigment dopedlayer, in which a small amount of pigment molecule having highfluorescent emission property is doped in the host material which is amain component thereof, is proposed as one constituent in configurationof a luminou layer (C. W. Tang, S. A. VanSlyke, and C. H. Chen, AppliedPhysics Letters, vol. 65, 3610 (1989)).

Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No.7-138739

Patent Document 2: JP-A No. 8-120442

Patent Document 3: JP-A No. 11-92915

Patent Document 4: JP-A No. 9-256142

Patent Document 5: JP-A No. 2003-313654

SUMMARY OF THE INVENTION

At the present, various element structures and organic materials areproposed for organic EL elements, and luminance of about 1,000 cd/m² isachieved at driving voltage of 10 V.

This value of luminance is, however, one that is obtained upon start ofdriving, and when the organic EL element is continuously driven, theoptical output drops and the driving voltage elevates with the course oftime, and the element is short-circuited at a certain stage due to thehigh driving voltage, and the organic EL element is broken.

Such rapid deterioration of an organic EL element is known to occurparticularly when oxygen, water or other foreign matter is present inthe organic EL element. Therefore, in a display device using such aconventional organic EL element, the display region composed of sequenceof organic EL element 10 is covered with an airtight cover 17, as shownin FIG. 2, and the inside of airtight cover 17 is packed with drynitrogen or the like so as to suppress deterioration of organicmaterial.

In such a configuration, however, although invasion of foreign matterfrom outside after assembly of the display device can be prevented,deterioration due to a foreign matter taken into the organic materialduring the manufacture of the organic EL element cannot be prevented.

The invention provides a method for manufacturing an organic EL elementcapable of suppressing intake of foreign matters into the elementstructure during manufacture of the organic EL element, so as to realizea longer element life, and an organic EL element manufactured by themanufacturing method.

One aspect of the invention is a method for manufacturing an organic ELelement including a substrate and a lamination structure formed oforganic films on the substrate, the laminated structure including atleast a hole transport layer, a light emitting layer, and an electrontransport layer, the method comprising: depositing each of the organicfilms composing the lamination structure, wherein at least a part of thedepositing is executed in the presence in the vicinity of the substrateof a metal element having reactivity to oxygen or water.

Another aspect of the invention is an organic EL display panelcomprising: a transparent substrate; a sequence of a plurality oforganic EL elements, provided to cover the transparent substrate; and anairtight cover, provided over the transparent electrode so as to coverthe plurality of organic EL elements so as to form a space filled withan inert gas; wherein each organic EL element comprises, between apositive electrode and a negative electrode, a lamination structureformed of organic films comprising at least a light emitting layer, ahole transport layer adjacent to the positive electrode side of thelight emitting layer, and an electron transport layer adjacent to thenegative electrode side of the light emitting layer, and wherein atleast one of the organic films composing the lamination structureincludes a metal element having reactivity to oxygen or water.

According to the invention, when depositing each organic film composingthe lamination structure, a metal element having reactivity to oxygenand water is disposed near the substrate on which the organic film isdeposited, impurities such as oxygen and water which are taken into thelamination structure in a case where the metal element is not present,is removed in the manufacturing process of organic EL element, and theremoved impurities are not returned to the lamination structure. Theorganic EL element thus manufactured has a feature of having the metalelement in the lamination structure, and hence has a long life.

Other objects and features of the invention will be more clearlyunderstood from the following detailed description of the inventiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a configuration of a conventional organic ELelement;

FIG. 2 is a diagram of a configuration of a conventional organic ELdisplay panel;

FIG. 3 is a diagram of a configuration of a vacuum deposition apparatusused in the invention;

FIG. 4 is a diagram of a configuration of an organic EL element in afirst embodiment of the invention;

FIG. 5 is a diagram of a manufacturing method of an organic EL elementin the first embodiment of the invention;

FIG. 6 is a diagram of a manufacturing method of an organic EL elementin a second embodiment of the invention; and

FIG. 7 is a diagram of a configuration of an organic EL display panel ina third embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 3 shows a configuration of a vacuum deposition apparatus 20 used inthe invention.

As shown in FIG. 3, the vacuum deposition apparatus 20 has a vacuum tank21 which is typically to be evacuated to a pressure of 1×10⁻⁴ to 1×10⁻⁵Pa from a exhaust port 21A connected to a high vacuum exhaust systemsuch as a rotary pump, a cryo pump, or a turbo molecular pump. Asubstrate holding bench 21B for holding a substrate W to be processed isprovided in the vacuum tank 21. Further, a cell 21C, which holds theorganic raw material, is provided oppositely to the substrate W to beprocessed on the substrate holding bench 21 in the vacuum tank 21.

A shutter mechanism 21D cooperates with the cell 21C, and a heatingmechanism 21F for electrically heating a Ti wire 21E is provided nearthe substrate W.

A substrate delivery/discharge port 21H having a gate valve 21G isformed in the vacuum tank 21.

When the vacuum deposition apparatus is in operation, the cell 21C isheated to a certain temperature, and the shutter mechanism 21D is openedso that a desired film such as organic film deposits on the surface ofthe substrate W to be processed on the substrate holding bench 21B.

At this time, in the vacuum deposition apparatus 20 in FIG. 3, theheating mechanism 21F is driven so that Ti atoms are discharged from theTi wire 21E to the inside of the vacuum tank 21. The discharged Ti atomsreact with oxygen or water, which are discharged to the inside of thevacuum tank 21 as a result of evaporation of organic materials from thecell 21C, oxygen or water, which remain in the vacuum tank 21, or oxygenor water, which stick to the wall of the vacuum tank 21, and inactivatethem. namely, the Ti atoms act as getter metal elements.

FIG. 4 shows a configuration of an organic EL element 40 in the firstembodiment of the invention manufactured by the vacuum depositionapparatus 20 in FIG. 3.

In FIG. 4, the organic EL element 40 is formed on a glass substrate 41which carries an ITO electrode pattern 41A (positive electrode). On theITO electrode pattern 41A, a hole injection layer 42, which is typicallyformed of a commercially-available 2-TNTA (4,4′,4″-tris (2-naphthylphenylamine) triphenylamine), which is expressed in the followingformula:

to have a film thickness of 140 nm, is formed. On the hole injectionlayer 42, a hole transport layer 43, which is typically formed of acommercially-available α-NPD (N,N-dinaphtyl-N,N-diphenyl[1,1′-biphenyl]-4,4′-diamine), which is expressed in the followingformula:

to have a film thickness of 10 nm , is formed. On the hole transportlayer 43, a light emitting layer 44, which is typically formed of acommercially-available CBP (4,4′-bis (9-carbazolyl)-(biphenyl)), whichis expressed in the following formula:

to have a film thickness of 20 nm, is formed. On the light emittinglayer 44, a hole blocking layer 45, which is typically formed of acommercially-available BAlq manufactured by SynTec GmbH Wolfen, which isexpressed in the following formula:

to have a film thickness of 10 nm, is formed. On the hole blocking layer45, an electron transport layer 46, which is typically formed of acommercially-available Alq₃, which is expressed in the followingformula:

to have a film thickness of 20 nm, is formed. On the electron transportlayer 46, an LiF electron injection layer 47, which is typically formedto have a film thickness of 0.5 nm. On the electron injection layer 47,an Al electrode 48 which is formed to have a film thickness of 100 mmThe hole transport layer 42 is doped by an acceptor of acommercially-available F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8 tetracyanoquinodimethane) manufactured by Tokyo Kasei Co. expressed in thefollowing formula at a concentration of 0.1%.

Further, the light emitting layer 44 is doped by tbppy(1,3,6,8-tetrabiphenyl pyrene) expressed in the following formula atconcentration of 10%.

The organic EL element 40 in FIG. 4 emits light at driving voltage of 4V or more, and radiates a blue light.

A method of manufacturing the organic EL element shown in FIG. 4including using the vacuum deposition apparatus 20 in FIG. 3 isexplained below.

FIG. 5 is a flowchart explaining the method of manufacturing the organicEL element shown in FIG. 4.

In FIG. 5, first at step 1, the glass substrate forming the ITO electronpattern 41A at a thickness of 150 nm is cleaned ultrasonically byacetone and isopropyl alcohol, and the cleaned substrate is treated byUV ozone or oxygen plasma. At step 2, it is put into the vacuum tank 21of the vacuum deposition apparatus 20 as the substrate W to beprocessed. In the case of UV ozone treatment, the substrate isirradiated with UV in atmosphere for 20 minutes.

At step 3, the vacuum tank 21 is evacuated to a high vacuum state of1×10⁻⁴ to 10⁻⁵ Pa, and at step 4, with the shutter 21D in FIG. 4 closed,and the cell 21C is heated by using resistance heater or electron beam,and the organic raw material contained in the cell 21 is dehydrated.

Further at step 4, the heating mechanism 21F is driven, and the Ti wire21E is heated, and Ti atoms are released into the vacuum tank 21. As aresult, oxygen and water released into the vacuum tank 21 by dehydrationof organic raw material at step 4 are captured by Ti atoms, and inert Tioxide or hydroxide is formed, and is fixed on the inner wall of thevacuum tank 21, thereby inhibiting return to the organic raw material,or absorption in the organic film formed on the substrate W to beprocessed.

At step 6, the shutter 21D is opened, and the deposition cell 21C isheated, and the hole injection layer 42, hole transport layer 43, lightemitting layer 44, hole blocking layer 45, electron transport layer 46,electron injection layer 47, and Al electrode layer 48 are formed on thesubstrate according to the composition in FIG. 4, sequentially by vacuumdeposition. For this purpose, a plurality of deposition cells 21Cholding the respective raw materials are disposed in the vacuum tank 21.In the deposition process, the substrate temperature of the substrate Wto be processed is set at room temperature.

More specifically, the hole injection layer 42 is formed by depositing2TNATA and F4-TCNQ at speeds of 0.1 nm/sec and 0.0001 nm/secrespectively, and the hole transport layer 43 is formed by depositingα-NPD at a speed of 0.1 nm/sec. The light emitting layer 44 is formed bydepositing CBP and tbppy at speeds of 0.09 nm/sec and 0.01 nm/secrespectively. The hole blocking layer 45 is formed by depositing BAlq atspeed of 0.1 nm/sec, and the electron transport layer 46 is formed bydepositing Alq₃ at speed of 0.1 nm/sec. The LiF electron injection layer47 and Al electrode layer are formed by depositing LiF and Al at speedsof 0.01 nm/sec and 1 nm/sec respectively.

The heating process of Ti wire 21E at step 5 is stopped immediatelybefore the start of step 6, and therefore in the deposition process atstep 6, Ti atoms are not discharged from the Ti wire 21E, but the Tiatoms discharged into the vacuum tank 21 at the preceding step 5 stillremain in the vacuum tank 21, and these atoms capture the oxygen orwater molecules existing near the surface of the substrate W to beprocessed, and inactivate them. Accordingly, even if raw materialparticles released at high temperature from the deposition cell 21C coolwhile moving through the vacuum tank 21 to near to the surface of thesubstrate W to be processed, incorporation of oxygen or water into theraw material particles can be effectively suppressed.

Comparative example 1 of the same composition as in FIG. 4 was formed inthe same way as the organic EL element of the embodiment but by omittingstep 5 in FIG. 5, that is, the step of discharge of Ti atoms into vacuumtank 21. In comparative example 2 of the embodiment, an organic ELelement of same composition as in FIG. 4 was formed by omitting step 4in FIG. 5, that is, the dehydrating process of organic raw material andomitting the process of step 5 in FIG. 5.

The organic EL element 40 of the embodiment, the organic EL element ofcomparative example 1, and the organic EL element of comparative example2 obtained in these manners were driven at a driving voltage of 7 V, anddriving current density of 15 mA/cm², and a high emission efficiency wasobtained just after the start of driving in all elements, and aftercontinuous driving for 200 hours, the relative luminance was comparedwith the luminance just after the start of driving, and as shown inTable 1, the relative luminance was 0.71 in the organic EL element 40 ofthe embodiment, but was lowered to 0.64 in comparative example 1, and0.38 in comparative example 2.

TABLE 1 Emission efficiency Relative luminance (cd/A) 15 mA/cm² 0 H 200H Comparative 3.15 1 0.64 example 1 Comparative 2.42 1 0.38 example 2Example 3.30 1 0.71

As seen from the results in Table 1, as in comparative example 2, if theraw material is not heated when forming each organic layer of organic ELelement by vacuum deposition process, oxygen or moisture in raw materialis incorporated into the element, with a fatal effect on the life oforganic EL element, or as in comparative example 1, when simply heated,the life of organic EL element is improved, but the life is furtherextended by inactivating the oxygen or moisture remaining in the vacuumtank, in addition to the dehydrating process, as in the embodiment. Inparticular, as compared with comparative example 1, the organic ELelement of the embodiment has a longer life, which suggests that if theorganic raw material is simply heated as at step 4 in FIG. 5 desorbedoxygen or moisture remaining in the vacuum tank 21 cannot be avoided,and is incorporated into the organic EL element on the substrate W to beprocessed, even if the vacuum tank 21 is evacuated to a high vacuumdegree by a high performance pump.

In the invention, since the Ti discharge process of step 5 is executedimmediately before deposition process at step 6, incorporation of Tiinto each layer of organic EL element 40 cannot be avoided, and as aresult of XPS analysis, indeed, it is confirmed that Ti is contained inthe organic EL element 40. However, as suggested by the results in Table1, as the emission efficiency is compared with the result right afterstart of driving, such presence of Ti does not have any adverse effecton the emission characteristics of the organic EL element, but theemission efficiency is shown to be improved, up to 3.30 cd/A, by thedecrease in impurities. In the organic EL elements of comparativeexample 1 and comparative example 2, since step 5 is omitted, it must benoted that Ti is not contained in the element structure. In spite ofthis fact, however, in these elements, the emission efficiency justafter the start of driving remains at 3.15 cd/A and 2.42 cd/A.

Thus, the invention is based on the finding that there is no adverseeffect on the emission characteristics if a metal element such as Ti iscontained in the organic EL element, and by using such a getter metalelement, the invention presents an organic EL element high in emissionefficiency and with reduced deterioration with aging, and an organic ELelement display panel using the same.

The metal is not limited to Ti alone, but similar effects are obtainedby using Si, Al, or Cr.

The invention is not limited to organic EL element of specific materialsystem, but is also effective for improving the element life in, forexample, the conventional organic EL element for emitting green lightshown in FIG. 1, or organic EL elements for emitting red, white or othercolor light.

In the vacuum deposition apparatus 20 in FIG. 3, heating of Ti wire 21Eis not limited to direct power feeding to the wire, but includes amethod of winding the Ti wire around a metal wire with a high meltingpoint such as W, and feeding power to the metal wire with high meltingpoint. The heating means of Ti wire 21E is not limited, and, forexample, a metal ribbon of wide area may be used instead of wire 21E.

In addition to the constitution above, by forming a large exposedsurface of the getter metal in the vacuum tank 21, free oxygen or waterin the vacuum tank 21 may be captured by the exposed surface.

Second Embodiment

FIG. 6 is a flowchart showing a method for manufacturing an organic ELelement in a second embodiment of the invention. In the diagram, thesame parts as in the above explanation are identified with samereference numerals, and the description thereof is omitted.

At step 6 in FIG. 5, all layers 42 to 48 are formed sequentially on anITO electrode of pattern 41A of FIG. 4, but in the process of theembodiment, in FIG. 6, after Ti discharge process at step 5, heating ofTi wire 41E is stopped at step 61, and only the hole injection layer 42is formed by the vacuum deposition apparatus 20 of FIG. 3, by openingthe shutters of the cell holding the 2-TNATA and the cell holding theF4-TCNQ. Then, at step 62, the Ti discharge process is executed again.

Further, at step 63 the Ti discharge process is terminated, and theshutter of the cell holding α-NPD is opened, and hole transport layer 43is formed, and by repeated alternate processing in a similar mannerthereafter the lamination structure of FIG. 4 is formed in sequence.

In the organic EL element formed in this manner, since Ti atoms aredischarged immediately before forming of each organic layer, andimpurities in the vacuum tank 21, especially oxygen and water near thesurface of the substrate W to be processed are removed, theconcentration of impurities incorporated into the organic EL element canbe further lowered.

As a result of these processes, the organic EL element manufactured inthis embodiment is higher in Ti concentration near the interfaces of onelayer with the next layer, and shows a characteristic Ti concentrationdistribution.

As mentioned above, in the invention, the metal used as getter is notlimited to Ti, and Si, Al, Cr and other metal may be used, and even byusing such metals, the emission characteristics of the organic ELelement do not deteriorate.

Third Embodiment

FIG. 7 shows a configuration of a full color organic EL display panel 60in a third embodiment of the invention.

In FIG. 7, the organic EL display panel 60 is formed on a glasssubstrate 61 carrying an ITO electrode pattern 61A, and includes asequence of display elements composed of repetition of an organic ELelement 60R which emits red light, an organic EL element 60G which emitsgreen light, and an organic EL element 60B which emits blue light. Theorganic EL elements 60R, 60G, 60B are formed in the process shown inFIG. 5 or FIG. 6 by using the vacuum deposition apparatus 20 shown inFIG. 3.

More specifically, the red organic EL element 60R has a laminationstructure formed on the ITO electrode pattern 61A. The laminationstructure has a hole injection layer 62R which is formed of α-NPD tohave a film thickness of 50 nm; a red light emitting layer 63R which isformed of Alq₃ which contains, at concentration of 1 wt. %, DCJTBexpressed in the following formula:

to have a film thickness of 30 nm; an electron transport layer 64R whichis formed of Alq₃ to have a film thickness of 30 nm; an electroninjection layer 65R which is formed of LiF to have a film thickness of0.5 nm; and an Al electrode layer 66R. Each one of the organic layers62R to 64R contains a getter metal element of any one of Ti, Si, Al, andCr. A concentration of the metal element is particularly high in thevicinity of the interface of one layer and another layer adjacentthereto.

Similarly, the green organic EL element 600 has a lamination structureformed on the ITO electrode pattern 61A. The lamination structure has ahole injection layer 62G which is formed of α-NPD to have a filmthickness of 50 nm; a green light emitting layer 63G which is formed ofAlq₃ to have a film thickness of 30 nm; an electron injection layer 64Gwhich is formed of LiF to have a film thickness of 0.5 nm; and an Alelectrode layer 650. The organic layers 62G and 63G contain a gettermetal element of any one of Ti, Si, Al, and Cr. A concentration of themetal element is particularly high in the vicinity of the interface ofone layer and another layer adjacent thereto.

The blue organic EL element 60B has a lamination structure formed on theITO electrode pattern 61A. The lamination structure has a hole injectionlayer 62B which is formed of α-NPD to have a film thickness of 50 nm; ablue light emitting layer 63B which is formed of CBP which contains, atconcentration of 10 wt. %, tppy expressed in the following formula:

to have a film thickness of 20 nm; a hole blocking layer 64B which isformed of BCP expressed in the following formula:

to have a film thickness of 10 nm; an electron transport layer 65B whichis formed of Alq₃ to have a film thickness of 50 nm; an electroninjection layer 66B which is formed of LiF to have a film thickness of0.5 nm; and an Al electrode layer 67B. Each one of the organic layers62B to 65B contains a getter metal element of any one of Ti, Si, Al, andCr. A concentration of the metal element is particularly high in thevicinity of the interface of one layer and another layer adjacentthereto.

All organic EL elements on the substrate 61 are covered with an airtightcover 68, and the inside of the airtight cover 68 is filled with aninert gas such as dry nitrogen gas.

In the organic EL element having such a configuration, the life ofindividual organic EL display elements is long due to the use of thegetter metal element, and a practical life for the whole of the displaypanel is obtained.

While the invention are herein described based on the preferredembodiments, the invention is not limited to these specific embodiments,and may be changed and modified in various manners within the scope ofthe invention described in the appended claims.

INDUSTRIAL APPLICABILITY

According to the invention, when depositing each organic film forcomposing the lamination structure, near the substrate for depositingthe organic films, a metal element reactive to oxygen or water ispresent, and therefore the oxygen or water taken into the laminationstructure are removed at the time of manufacture of organic EL element,and removed impurities do not return to the lamination structure. Theorganic EL element thus manufactured includes the metal element in thelamination structure, and has an excellent element life.

1. A method for manufacturing an organic EL element including asubstrate and a lamination structure formed of organic films on thesubstrate, the laminated structure including at least a hole transportlayer, a light emitting layer, and an electron transport layer, themethod comprising: depositing each of the organic films composing thelamination structure, wherein at least a part of the depositing isexecuted in the presence in the vicinity of the substrate of a metalelement having reactivity to oxygen or water.
 2. The method formanufacturing an organic EL element of claim 1, wherein the metalelement comprises one selected from Ti, Si, Al, and Cr.
 3. The methodfor manufacturing an organic EL element of claim 1, wherein thedepositing is preceded by discharging the metal element to the vicinityof the substrate.
 4. The method for manufacturing an organic EL elementof claim 3, wherein the discharging of the metal element is terminatedbefore the depositing starts.
 5. The method for manufacturing an organicEL element of claim 3, wherein the discharging of the metal elementcomprises heating a medium comprising the metal element.
 6. The methodfor manufacturing an organic EL element of claim 3, wherein thedischarging of the metal element is executed in advance to thedepositing of one of the organic films composing the laminationstructure.
 7. The method for manufacturing an organic EL element ofclaim 3, wherein the discharging of the metal element is executed inadvance of the depositing of each organic film composing the laminationstructure.